1. Field of the Invention
The present invention relates to mechanisms for securing a nuclear reactor fuel assembly to its support and, in particular, a spring driven mechanism for avoiding levitation effects caused by coolant flow in the fuel assembly.
2. Background of the Related Art
It has long been known that the coolant flow rate in modern nuclear reactors can be so large that the weight of the fuel assembly is insufficient to prevent the assembly from being lifted off the core support stand. One solution to overcome this problem is shown in FIG. 1. There, hold-down springs 16 are provided between the upper tie plate 12 of the reactor core and the upper support plate 14. Force provided by the hold-down springs is designed to overcome the levitation force created by the coolant flow through the fuel assembly. Without the springs, the fuel assembly will levitate, a condition which can result in fretting damage to the fuel assembly and the location pins 2 in the reactor upper and lower core support plates.
Under normal operation, the downward force of the hold-down springs is greater than the upward levitation force and the fuel assembly 10 remains seated on the lower core support plate 15. Whenever the levitation force exceeds the hold-down force, however, levitation occurs. This happens during coolant pump overspeed events and could also happen if all cooling pumps were operated during cold conditions. It should be noted here that the higher density of the cold coolant results in higher levitation forces. It should also be noted that there are limitations on the allowable force that may be applied to the reactor upper core plate which prevent incorporation of stiff springs which would hold the fuel assembly down under all conditions.
In the known hold-down design, there is nothing that can be done to prevent levitation during a pump overspeed event but reactor operational limitations are imposed on the allowable coolant flow through the assembly during start up from a cold shut down. The known procedure involves starting the coolant pumps in sequence as the coolant heats up. Such cold start-up flow limitations increase the time it takes for the reactor to reach full temperature and power by several hours. Economic and other operational considerations make it desirable for the reactor to be able to reach full power as quickly as possible.
This known use of hold-down mechanisms for overcoming the coolant flow levitation effect is indicated by a multiplicity of patents with variations on this theme. U.S. Pat. Nos. 4,278,501, 4,534,933, 4,671,924 and 4,986,960 are typical of a number of these hold-down approaches discussed above. These approaches employ the concept of holding down the nuclear fuel assembly by a force which is opposite to the direction of the levitation force created by the coolant flow. All of these prior art approaches are limited in various respects since there may always be a levitation force created by coolant flow which exceeds the hold-down force provided by the hold-down mechanism. There have also been attempts to employ other means for holding down the nuclear fuel assembly such as the use of a latch device oriented in the lower support of the fuel assembly as disclosed in U.S. Pat. No. 4,309,251. Such device is only applicable to C. E. type fuel with a large central tube to contain the latch operating push rod. This approach also requires a notched reactor pin. Such pins are normally not notchable. It would be prohibitively expensive to utilize such design for retrofitting purposes.